Optical device comprising a charge transport layer of insoluble organic material and method for the production thereof

ABSTRACT

A method of forming an organic light emitting diode comprising the steps of: providing a substrate comprising a first electrode for injection of charge carriers of a first type; forming a charge transporting layer by depositing over the substrate a charge transporting material for transporting charge carriers of the first type, the charge transporting material being soluble in a solvent; treating the charge transporting layer to render it insoluble in the solvent; forming an electroluminescent layer by depositing onto the charge transporting layer a composition comprising the solvent, a phosphorescent material, and a host material; and depositing over the electroluminescent layer a second electrode for injection of charge carriers of a second type.

FIELD OF THE INVENTION

This invention relates to organic optical devices comprising a layer ofinsoluble organic material and methods for the production thereof.

BACKGROUND OF THE INVENTION

Organic semiconductors are now frequently used in a number of opticaldevices such as in organic light emitting diodes (“OLEDs”) as disclosedin WO 90/13148, photovoltaic devices as disclosed in WO 96/16449 andphotodetectors as disclosed in U.S. Pat. No. 5,523,555.

A typical OLED comprises a substrate, on which is supported an anode(commonly indium tin oxide or “ITO”), a cathode and an organicelectroluminescent layer between the anode and cathode. In operation,holes are injected into the device through the anode and electrons areinjected into the device through the cathode. The holes and electronscombine in the organic electroluminescent layer to form an exciton whichthen undergoes radiative decay to give light. Other layers may bepresent in the OLED, for example a layer of organic hole injectionmaterial such as poly(ethylene dioxy thiophene)/polystyrene sulfonate(PEDT/PSS) may be provided between the anode and the organicelectroluminescent layer to assist injection of holes from the anode tothe organic electroluminescent layer.

Various classes of organic light emitting materials are known, inparticular: polymers such as poly(p-phenylenevinylene) (as disclosed inWO 90/13148), polyfluorenes and polyphenylenes; the class of materialsknown as small molecule materials such astris-(8-hydroxyquinoline)aluminium (“Alq₃”) as disclosed in U.S. Pat.No. 4,539,507; and the class of materials known as dendrimers asdisclosed in WO 99/21935. These materials electroluminesce by radiativedecay of singlet excitons (i.e. fluorescence) however spin statisticsdictate that up to 75% of excitons are triplet excitons which undergonon-radiative decay, i.e. quantum efficiency may be as low as 25% forfluorescent OLEDs—see, for example, Chem. Phys. Lett., 1993, 210, 61,Nature (London), 2001, 409, 494, Synth. Met, 2002, 125, 55 andreferences therein.

Accordingly, considerable effort has been directed towards producingluminescence from triplet excitons (phosphorescence) by utilisingspin-orbit coupling effects in metal complexes that enable tripletexcitons to undergo radiative decay. The metal complex is doped into ahost material from which it receives charge and/or triplet excitons.Examples of complexes investigated for this purpose include lanthanidemetal chelates [Adv. Mater., 1999, 11, 1349], a platinum (II) porphyrin[Nature (London), 1998, 395, 151] and tris(phenylpyridine) iridium (III)(hereinafter “Ir(ppy)₃”) [Appl. Phys. Lett., 1999, 75, 4; Appl. Phys.Lett., 2000, 77, 904]. Fuller reviews of such complexes may be found inPure Appl. Chem., 1999, 71, 2095, Materials Science & Engineering, R:Reports (2002), R39(5-6), 143-222 and Polymeric Materials Science andEngineering (2000), 83, 202-203.

Prior art phosphorescent OLEDs often comprise charge transporting and/orcharge blocking layers used in conjunction with the electroluminescentlayer in order to maximise device efficiency. The chargetransporting/blocking layers and the electroluminescent layers aretypically formed by vacuum evaporation of the appropriate materials insequence.

Deposition of materials from solution, for example by spin-coating orinkjet printing, offers advantages over vacuum deposition such assimplified processing. However, solution deposition of multiple layersis complicated by the tendency of initially cast or deposited layers todissolve in the solvents used for succeeding layers. Thus, solutiondeposition will commonly be employed for one layer only. For example, US2002/096995 discloses spin-coating of an electroluminescent layer ofpolyvinylcarbazole (hereinafter “PVK”) host doped with Ir(ppy)₃ emitteronto a substrate of ITO and PEDT/PSS. A layer of electron transportingmaterial is then deposited by vacuum evaporation.

One solution to this problem is disclosed in JP 2003-077673 wherein ahole transporting layer of PVK is formed by spin-coating from1,2-dichloroethane solution followed by formation of anelectroluminescent layer of 9,9-dioctylfluorene host/Ir(ppy)₃ emitter byspin-coating from xylene solution. The formation of this bilayer bysolution processing is possible due to the low solubility of PVK in thexylene solvent used for the electroluminescent layer. This approach islimited in that the material used for the first layer can only beselected from those that do not dissolve in the solvent used for thesecond layer.

Another solution to this problem is disclosed in JP 2002-050482 whereina first layer of insoluble poly(phenylenevinylene) (hereinafter “PPV”)is formed by deposition by spin-coating of a soluble precursor compound,followed by thermal conversion of the precursor to insoluble PPV. Alayer of PVK host/Ir(ppy)₃ guest is then deposited onto the insolublePPV layer. Again, this approach is limited in that it is only applicableto insoluble compounds that have a soluble precursor form. Furthermore,the thermal conversion required by these precursors requires forcingconditions and generates corrosive by-products that may harm theperformance of the finished device.

It is therefore an object of the invention to provide a method offorming a phosphorescent OLED by solution deposition of a plurality oflayers that is not constrained by the nature of the active materialbeing used. It is a further object of the invention to provide aphosphorescent OLED having improved device performance.

SUMMARY OF THE INVENTION

The present inventors have found that a phosphorescent OLED comprising aplurality of solution processable layers may be formed by treatment of asolution processable layer to render it insoluble.

Accordingly, in a first aspect the invention provides a method offorming an organic light emitting diode comprising the steps of:

-   -   providing a substrate comprising a first electrode for injection        of charge carriers of a first type    -   forming a charge transporting layer by depositing over the        substrate a charge transporting material for transporting charge        carriers of the first type, the charge transporting material        being soluble in a solvent;    -   treatment of the charge transporting layer to render it        insoluble in the solvent;    -   forming an electroluminescent layer by depositing onto the        charge transporting layer a composition comprising the solvent,        a phosphorescent material and a host material; and    -   depositing over the electroluminescent layer a second electrode        for injection of charge carriers of a second type.

Preferably, the first electrode is an anode; the second electrode is acathode; the charge carriers of the first type are holes; and the chargecarriers of the second type are electrons.

Preferably, the charge transporting material is deposited from solution.The solvent used for such a solution may be the same as or different tothe solvent used to deposit the electroluminescent layer.

In one embodiment, the charge transporting material comprises across-linkable material and the treatment comprises subjecting thecharge transporting layer to heat or electromagnetic radiation, inparticular UV radiation, in order to cross-link the charge transportingmaterial.

The present inventors have surprisingly found that the chargetransporting layer may be rendered insoluble in the solvent even whencross-linking groups are absent. Accordingly, in another embodiment thecharge transporting layer is substantially free of cross-linkable groupsand the treatment comprises subjecting the charge transporting layer toheat.

Following the aforementioned treatment, it may be possible to solubilisethe charge transporting layer under forcing conditions (e.g. by exposureto the solvent at elevated temperature and/or immersion in the solventover a prolonged period), particularly in the case where the chargetransporting layer is substantially free of cross-linkable groups. Inthis case, the present inventors have found that at least part of thecharge transporting layer remains intact under conditions typicallyemployed for deposition of the electroluminescent layer, although anupper part of the charge transporting layer may dissolve upon contactwith the solvent. The term “insoluble” should be construed accordingly.

Partial dissolution of the charge transporting layer may be desirable inorder to form a mixed region between the charge transporting layer andthe electroluminescent layer. The extent of this partial dissolution maybe controlled by appropriate selection of conditions forinsolubilisation of the charge transporting layer. Thus, use ofcross-linking groups will typically result in insolubilisation of thewhole of the charge transporting layer as deposited. Alternatively,selection of time and temperature for heat treatment of a chargetransporting layer that is substantially free of cross-linkable groupsmay be used to control the extent of insolubilisation of the chargetransporting layer as deposited.

Preferably, the charge transporting material is a polymer.

Preferably, the polymer comprises an optionally substituted triarylaminerepeat unit.

Preferably, the triarylamine repeat unit comprises an optionallysubstituted first repeat unit of formula (I):

wherein each Ar¹, Ar² and Ar³ is the same or different and independentlyrepresents optionally substituted aryl or heteroaryl; and n is 0 or 1.

Preferably, Ar¹ is phenylene.

Preferably, Ar² is 1,4-phenylene or 4,4′-biphenylene. When n is 0, Ar²is preferably 1,4-phenylene. When n is 1, Ar² is preferably1,4-phenylene or 4,4′-biphenylene.

Preferably, Ar³ is phenyl. Ar³ may be substituted or unsubstituted;preferably Ar³ is substituted with a solubilising group, more preferablyoptionally substituted C₁₋₂₀ alkyl or C₁₋₂₀ alkoxy.

Preferably, the polymer is a copolymer comprising two or more repeatunits. Preferably, one of the repeat units of such a copolymer is thefirst repeat unit of formula (I). More preferably, the copolymercomprises a second repeat unit selected from optionally substitutedfluorene, indenofluorene, spirofluorene and phenylene.

Preferably, the phosphorescent material is a metal complex.

Preferably, the host material is a host polymer.

Preferably, the host polymer is a polymer comprising a triarylaminerepeat unit as defined above, preferably a first repeat unit.

In a second aspect, the invention provides an organic light emittingdiode obtainable by the method according to the first aspect of theinvention.

In a second aspect, the invention provides an organic light emittingdiode comprising, in sequence, an anode; a hole transporting layer; anelectroluminescent layer comprising a phosphorescent material and a hostmaterial; and a cathode, wherein the hole transporting layer is apolymer comprising an optionally substituted repeat unit of formula (I):

wherein each Ar¹, Ar² and Ar³ is the same or different and independentlyrepresents optionally substituted aryl; and n is 0 or 1.

Preferably, the polymer comprises a repeat unit selected from optionallysubstituted fluorene, indenofluorene, spirofluorene and phenylene.

Preferably, a hole injecting layer comprising a conductive organicmaterial is located between the anode and the hole transporting layer.

Preferably, the phosphorescent material is a metal complex.

In a third aspect, the invention provides an organic light emittingdiode comprising a first electrode for injection of charge carriers of afirst type; a second electrode for injection of charge carriers of asecond type; an electroluminescent layer between the first and secondelectrodes comprising a phosphorescent material and a host material;wherein a mixed layer comprising a mixture of a charge transportingmaterial for transporting charge carriers of the first type, thephosphorescent material and the host material is located between thecharge transporting layer and the electroluminescent layer.

Preferably, the mixed layer comprises a concentration gradient acrossits thickness wherein the concentration of charge transporting materialdecreases with distance from the first electrode.

Preferably, the first electrode is an anode, the second electrode is acathode and the charge transporting material is a hole transportingmaterial.

In a fourth aspect, the invention provides a method of forming a deviceaccording to the third aspect of the invention comprising the steps of:

-   -   providing a substrate comprising a first electrode for injection        of charge carriers of a first type;    -   depositing over the substrate a charge transporting material for        transporting charge carriers of the first type, the charge        transporting material being soluble in a solvent;    -   forming an electroluminescent layer by depositing onto the        charge transporting layer a composition comprising the solvent,        a phosphorescent material and a host material; and    -   depositing over the electroluminescent layer a second electrode        for injection of charge carriers of a second type.

The method of the fourth aspect will result in at least partialdissolution of the charge transporting material upon deposition of theelectroluminescent layer.

The extent to which the charge transporting material dissolves may becontrolled by appropriate treatment of the charge transporting materialfollowing its deposition as described above with respect to dissolutionof the charge transporting layer of the first aspect of the invention.

Preferably, the first electrode is an anode, the second electrode is acathode and the charge transporting material is a hole transportingmaterial.

In a fifth aspect, the invention provides a method of forming a deviceaccording to the third aspect of the invention comprising the steps of:

-   -   providing a substrate comprising a first electrode for injection        of charge carriers of a first type;    -   depositing over the substrate a composition comprising a        solvent, a phosphorescent material, a host material and charge        transporting material for transporting charge carriers of the        first type;    -   phase-separating the composition during evaporation of the        solvent such that the charge transporting material migrates        towards the first electrode; and    -   depositing over the electroluminescent layer a second electrode        for injection of charge carriers of a second type.

The method according to the fifth aspect provides a method of forming adevice comprising a mixed region and/or a charge transporting layer.Preferably, the phase separation of the charge transporting material,electroluminescent material and host material results in mixed regioncomprising a concentration gradient across its thickness wherein theconcentration of charge transporting material decreases with distancefrom the first electrode.

Preferably, the first electrode is an anode; the second electrode is acathode; and the charge transporting material is a hole transportingmaterial.

Preferably, the phosphorescent material and host material according tothe fifth aspect are components of the same molecule.

Preferably, the composition is deposited onto a layer of conductiveorganic material provided over the first electrode. Preferably, theconductive organic material comprises dopedpoly(ethylenedioxythiophene), more preferablypoly(ethylenedioxythiophene) doped with poly(styrene sulfonate).

Preferably, the charge transporting material is a polymer, morepreferably a polymer comprising a triarylamine repeat unit, mostpreferably a triarylamine repeat unit of formula (I).

The present inventors have found that phosphorescence of aphosphorescent OLED may be quenched if the electroluminescent layer isadjacent to a low triplet energy level charge transporting material.

Accordingly, in a sixth aspect, the invention provides an organic lightemitting diode comprising, in sequence, a first electrode for injectionof charge carriers of a first type; a first charge transporting layercomprising a first charge transporting material for transporting chargecarriers of the first type; an electroluminescent layer adjacent to thecharge transporting layer and comprising a phosphorescent material and ahost material; and a second electrode for injection of charge carriersof a second type, wherein the first charge transporting material has ahigher triplet energy level than the phosphorescent material.

Although low triplet energy level charge transporting or injectingmaterials may be undesirable if placed adjacent to theelectroluminescent layer, such materials may serve to optimise deviceperformance if they are placed remote from the electroluminescent layer.

Accordingly, it is preferred that a second charge transporting layer fortransport of charge carriers of the first type is provided between thefirst electrode and the first charge transporting layer.

Similarly, it may be desirable to provide a charge injecting layercomprising a conductive organic material for injecting charge carriersof the first type remote from the electroluminescent layer to preventany quenching that may occur if these layers are adjacent (e.g.quenching by a polaron band of the conductive organic material).

Accordingly, it is preferred that a conductive organic material forinjection of charge carriers of the first type is provided between thefirst electrode and the first charge transporting layer.

Preferably, the conductive organic material comprises dopedpoly(ethylenedioxythiophene) more preferablypoly(ethylenedioxythiophene) doped with poly(styrene sulfonate).

Preferred charge transporting materials, host materials andelectroluminescent materials for the third, fourth, fifth and sixthaspects of the invention are as described with respect to the firstaspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 shows an OLED prepared according to the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, an organic light emitting diode preparedaccording to the method of the invention comprises a substrate 1, ananode 2 of indium tin oxide, a layer 3 of organic hole injectionmaterial, an insoluble hole transporting layer 4, an electroluminescentlayer 5 and a cathode 6.

Optical devices tend to be sensitive to moisture and oxygen.Accordingly, the substrate 1 preferably has good barrier properties forprevention of ingress of moisture and oxygen into the device. Thesubstrate is commonly glass, however alternative substrates may be used,in particular where flexibility of the device is desirable. For example,the substrate may comprise a plastic as in U.S. Pat. No. 6,268,695 whichdiscloses a substrate of alternating plastic and barrier layers or alaminate of thin glass and plastic as disclosed in EP 0949850.

Although not essential, the presence of layer 3 of organic holeinjection material is desirable as it assists hole injection from theanode into the layer or layers of semiconducting polymer. Examples oforganic hole injection materials include PEDT/PSS as disclosed in EP0901176 and EP 0947123, or polyaniline as disclosed in U.S. Pat. No.5,723,873 and U.S. Pat. No. 5,798,170.

Cathode 6 is selected in order that electrons are efficiently injectedinto the device and as such may comprise a single conductive materialsuch as a layer of aluminium. Alternatively, it may comprise a pluralityof metals, for example a bilayer of calcium and aluminium as disclosedin WO 98/10621. A thin layer of dielectric material such as lithiumfluoride may be provided between the electroluminescent layer 5 and thecathode 6 to assist electron injection as disclosed in, for example, WO00/48258.

The device is preferably encapsulated with an encapsulant (not shown) toprevent ingress of moisture and oxygen. Suitable encapsulants include asheet of glass, films having suitable barrier properties such asalternating stacks of polymer and dielectric as disclosed in, forexample, WO 01/81649 or an airtight container, optionally with adesiccant, as disclosed in, for example, WO 01/19142.

In a practical device, at least one of the electrodes issemi-transparent in order that light may be emitted. Where the anode istransparent, it typically comprises indium tin oxide. Examples oftransparent cathodes are disclosed in, for example, GB 2348316.

A typical electroluminescent device comprises an anode having a workfunction of 4.8 eV. Accordingly, the HOMO level of the hole transportingmaterial for hole transporting layer 4 is preferably around 4.8-5.5 eV.Suitable hole transporting materials are small molecules, polymers anddendrimers comprising optionally substituted triarylamine units, inparticular triphenylamine units, and materials comprising carbazoleunits.

The hole transporting layer 4 may also serve as an electron blockinglayer. Electron blocking functionality is provided if the holetransporting layer 4 possesses a shallower (i.e. less positive) LUMOlevel(s) than the host material and the phosphorescent dopant. For atypical device architecture, suitable LUMO level for the holetransporting layer 4 is about 1.6-2.3 eV.

The hole transporting layer 4 may also serve as an exciton blockinglayer. Exciton blocking functionality is provided if the holetransporting layer 4 has (a) a wider HOMO-LUMO bandgap and (b) a widerT₁-T₀ energy gap than the phosphorescent dopant.

Preferred hole transporting materials for hole transporting layer 4 arepolymers comprising triarylamine repeat units such as polymers havingrepeat units 1-6:

wherein X, Y, A, B, C and D are independently selected from H or asubstituent group. More preferably, one or more of X, Y, A, B, C and Dis independently selected from the group consisting of optionallysubstituted, branched or linear alkyl, aryl, perfluoroalkyl, thioalkyl,cyano, alkoxy, heteroaryl, alkylaryl and arylalkyl groups. Mostpreferably, X, Y, A and B are C₁₋₁₀ alkyl. The repeat unit of formula 1is most preferred.

These polymers may be homopolymers or copolymers. Where they arecopolymers, suitable co-repeat units are optionally substituted arylenessuch as fluorenes as disclosed in Adv. Mater. 2000 12(23) 1737-1750,particularly 2,7-linked 9,9-dialkyl fluorene, 9,9-diaryl fluorene or9-alkyl-9-aryl fluorene; a spirofluorene such as 2,7-linkedspirobifluorene as disclosed in EP 0707020; an indenofluorene such as a2,7-linked indenofluorene; or a phenylene such as alkyl or alkoxysubstituted 1,4-phenylene. Each of these repeat units may besubstituted. These copolymers are particularly advantageous because theyhave good film forming properties and may be readily formed by Suzuki orYamamoto polymerisation which enables a high degree of control over theregioregularity of the resultant polymer.

Preferred polyfluorenes comprise repeat units of formula (II):

wherein R and R′ are independently selected from hydrogen or optionallysubstituted alkyl, alkoxy, aryl, arylalkyl, heteroaryl andheteroarylalkyl, and at least one of R and R′ is not hydrogen. Morepreferably, at least one of R and R′ comprises an optionally substitutedC₁-C₂₀ alkyl group, more preferably C₄-C₁₀ alkyl group.

Preferably, the first semiconducting polymer comprises a 1:1 regular,alternating copolymer of a fluorene repeat unit and a triarylaminerepeat unit.

Another suitable hole transporting polymer is poly(vinyl carbazole).

To avoid any quenching of phosphorescence by the hole transporting layerwhen used with high triplet energy materials such as greenphosphorescent materials, the hole transporting polymer may be selectedso as to have repeat units that increase the triplet energy of thepolymer as compared to repeat units of formula (II). Suitable repeatunits of this type include non-planar repeat units and repeat units thatare partially or fully non-conjugated.

Non-planar repeat units include units containing a twist caused bysteric interaction between a substituent of the non-planar repeat unitand ring bearing substituents capable of inducing a twist betweenadjacent ring systems by steric interaction.

One class of repeat unit capable of producing a twist in the polymerbackbone is that comprising an optionally substituted repeat unit offormula (III):

wherein m is 0, 1 or 2 and R³ is a substituent, preferably an optionallysubstituted alkyl, alkoxy, aryl, aryloxy, heteroaryl or heteroaryloxygroup, more preferably a C₁₋₁₀ alkyl group.

The group R³ induces a twist by steric interaction with the phenyl groupof the repeat unit of formula (III) adjacent to the phenyl group that R³is attached to. In this case, preferred repeat units include repeatunits of formulae (IV), (V) or (VI):

Alternatively, the group R³ may induce a twist by steric interactionwith a repeat unit adjacent to the repeat unit of formula (III). In thiscase, R³ is attached to the carbon atom adjacent to the carbon atom ofthe repeat unit that is linked to said adjacent repeat unit.

Examples of partially or fully non-conjugated repeat units for highertriplet energy hole transporting polymers have formulae 7-10 (dottedlines show bonds for linkage to further repeat units). Repeat unitshaving an aryl group at either end are particularly advantageous becausethey are readily formed from the appropriate monomers by Suzuki orYamamoto polymerisation.

Another class of repeat units for high triplet energy polymers areoptionally substituted, 3,6-linked fluorene or carbazole repeat units.

The hole transporting material is soluble in at least some organicsolvents, in particular to allow its deposition from solution and/or toallow intermixing upon deposition of the electroluminescent layer fromsuch a solvent. Following deposition, the hole transporting material istreated so as to render it insoluble.

One suitable treatment entails cross-linking of cross-linkable groupsthat are either bound to or blended with the hole transporting material.Suitable cross-linkable groups include oxetanes as disclosed in Nature421, 829-833, 2003; azides; acrylates as disclosed in WO 96/20253; vinylgroups as disclosed in U.S. Pat. No. 6,107,452; and ethynyl groups asdisclosed in Kim et al., Synthetic Metals 122 (2001), 363-368.Cross-linking may be achieved through thermal treatment or exposure ofthe hole transporting layer to radiation, in particular UV radiation.

The hole transporting layer may be rendered insoluble even if nocross-linkable groups are present within it by an appropriateinsolubilisation treatment. The hole transporting layer is then stableto solution deposition of the electroluminescent layer. The presentinventors have found that this insoluble layer forms (a) regardless ofwhether a PEDT/PSS layer is used or not (although presence of PEDT/PSSis preferred) and (b) in air or in a nitrogen only environment. However,the present inventors have found that it is necessary to subject thehole transporting layer to an insolubilising treatment in order tomaximise device performance, and in particular to minimise mixing of thesecond layer with the first layer by increasing the insolubility of thefirst layer. Without wishing to be bound by any theory, possiblemechanisms for loss of solubility of the hole transporting layer uponinsolubilisation treatment when no cross-linkable groups are presentinclude formation of a relatively solvent impermeable surface or anadhesion to the surface that the hole transporting layer followingelimination of solvent from a solution deposited hole transportinglayer.

Means for eliminating solvent include vacuum treatment and/or heattreatment. Heat treatment, either alone or in combination with vacuumtreatment, is preferred. For heat treatment, a suitable treatment periodis in the range 5 minutes-2 hours. The heat treatment temperature ispreferably above the glass transition temperature of the holetransporting material.

Preferably, the hole transporting layer has a thickness in the range10-20 nm. If a hole transporting polymer is used to form the holetransporting layer 4, its thickness may be modified by appropriateselection of molecular weight (Mw) of the polymer. Thus, Mw of 50,000may give a thickness as low as about 2 nm, but up to about 15-20 nm forMw of around 250,000-300,000.

The electroluminescent layer 5 is deposited directly onto the holetransporting layer 4 and comprises a host material and a phosphorescentdopant. It is necessary for the host to possess a higher T₁ energy levelthan the dopant. Examples of suitable host materials are small moleculescomprising triarylamine units (for examples see Shirota, J. Mater.Chem., 2000, 10, 1-25); polymers comprising triarylamine repeat units asdescribed above; or carbazole units, in particular poly(vinylcarbazole).

The host material may also have charge transporting properties. Holetransporting host materials are particularly preferred such as theoptionally substituted hole-transporting arylamine having the followingformula:

wherein Ar⁵ is an optionally substituted aromatic group, such as phenyl,or

and Ar⁶, Ar⁷, Ar⁸ and Ar⁹ are optionally substituted aromatic orheteroaromatic groups (Shi et al (Kodak) U.S. Pat. No. 5,554,450. VanSlyke et al, U.S. Pat. No. 5,061,569. So et al (Motorola) U.S. Pat. No.5,853,905 (1997)). Ar is preferably biphenyl. At least two of Ar⁶, Ar⁷,Ar⁸ and Ar⁹ may be bonded to either a thiol group, or a group containinga reactive unsaturated carbon-carbon bond. Ar⁶ and Ar⁷ and/or Ar⁸ andAr⁹ are optionally linked to form a N containing ring, for example sothat the N forms part of a carbazole unit e.g.

Host materials may alternatively possess electron transportingproperties. Examples of electron transporting host materials are azoles,diazoles, triazoles, oxadiazoles, benzoxazoles, benzazoles andphenanthrolines, each of which may optionally be substituted.Particularly preferred substituents are aryl groups, in particularphenyl. oxadiazoles, in particular aryl-substituted oxadiazoles. Thesehost materials may exist in small molecule form or may be provided asrepeat units of a polymer, in particular as repeat units located in thebackbone of a polymer or as substituents pendant from a polymerbackbone. Specific examples of electron transporting host materialsinclude 3-phenyl-4-(1-naphthyl)-5-phenyl-1,2,4-triazole and 2,9-dimethyl-4,7-diphenyl-phenanthroline.

Host materials may be bipolar, i.e. capable of transporting holes andelectrons. Suitable bipolar materials preferably contain at least twocarbazole units (Shirota, J. Mater. Chem., 2000, 10, 1-25). In onepreferred compound, both Ar⁶ and Ar⁷ and Ar⁸ and Ar⁹ as described aboveare linked to form carbazole rings and Ar⁵ is phenyl. Alternatively, abipolar host material may be a material comprising a hole transportingsegment and an electron transporting segment. An example of such amaterial is a polymer comprising a hole transporting segment and anelectron transporting segment as disclosed in WO 00/55927 wherein holetransport is provided by a triarylamine repeat unit located within thepolymer backbone and electron transport is provided by a conjugatedpolyfluorene chain within the polymer backbone. Alternatively, theproperties of hole transport and electron transport may be provided byrepeat units pendant from a conjugated or non-conjugated polymerbackbone.

Specific examples of “small molecule” hosts include4,4′-bis(carbazol-9-yl)biphenyl), known as CBP, and(4,4′,4″-tris(carbazol-9-yl)triphenylamine), known as TCTA, disclosed inIkai et al. (Appl. Phys. Lett., 79 no. 2, 2001, 156); and triarylaminessuch as tris-4-(N-3-methylphenyl-N-phenyl)phenylamine, known as MTDATA.

Homopolymers and copolymers may be used as hosts, including optionallysubstituted polyarylenes such as polyfluorenes, polyspirofluorenes,polyindenofluorenes or polyphenylenes as described above with respect tothe hole transporting layer.

Specific examples of host polymers disclosed in the prior art includepoly(vinyl carbazole) disclosed in, for example, Appl. Phys. Lett. 2000,77(15), 2280; polyfluorenes in Synth. Met. 2001, 116, 379, Phys. Rev. B2001, 63, 235206 and Appl. Phys. Lett. 2003, 82(7), 1006;poly[4-(N-4-vinylbenzyloxyethyl,N-methylamino)-N-(2,5-di-tert-butylphenylnapthalimide] in Adv. Mater.1999, 11(4), 285; poly(para-phenylenes) in J. Mater. Chem. 2003, 13,50-55;poly[9,9′-di-n-hexyl-2,7-fluorene-alt-1,4-(2,5-di-n-hexyloxy)phenylene]as a host for both fac-tris(2-phenylpyridine) iridium(III) and2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) in J. Chem.Phys. (2003), 118(6), 2853-2864; a random copolymer host ofdioctylfluorene and dicyano-benzene in Mat. Res. Symp. Spring Meeting2003 Book of Abstracts, Heeger, p. 214; and an AB copolymer of afluorene repeat unit and phenylene repeat unit is disclosed in Mat. Res.Soc. Symp. Proc. 708, 2002, 131.

Suitable phosphorescent dopants are metal complexes based on heavyelements M that induce strong spin-orbit coupling to allow rapidintersystem crossing and emission from triplet states. Suitable heavymetals M include:

-   -   lanthanide metals such as cerium, samarium, europium, terbium,        dysprosium, thulium, erbium and neodymium; and    -   d-block metals, in particular those in rows 2 and 3 i.e.        elements 39 to 48 and 72 to 80, in particular ruthenium,        rhodium, palladium, rhenium, osmium, iridium, platinum and gold.

Suitable coordinating groups for the f-block metals include oxygen ornitrogen donor systems such as carboxylic acids, 1,3-diketonates,hydroxy carboxylic acids, Schiff bases including acyl phenols andiminoacyl groups. As is known, luminescent lanthanide metal complexesrequire sensitizing group(s) which have the triplet excited energy levelhigher than the first excited state of the metal ion. Emission is froman f-f transition of the metal and so the emission colour is determinedby the choice of the metal. The sharp emission is generally narrow,resulting in a pure colour emission useful for display applications.

The d-block metals form organometallic complexes with carbon or nitrogendonors such as porphyrin or bidentate ligands of formula (VII):

wherein Ar¹⁰ and Ar¹¹ may be the same or different and are independentlyselected from optionally substituted aryl or heteroaryl; X¹ and Y¹ maybe the same or different and are independently selected from carbon ornitrogen; and Ar¹⁰ and Ar¹¹ may be fused together. Ligands wherein X¹ iscarbon and Y¹ is nitrogen, or wherein X¹ and Y¹ are both nitrogen areparticularly preferred.

Examples of bidentate ligands are illustrated below:

One or both of Ar¹⁰ and Ar¹¹ may carry one or more substituents.Particularly preferred substituents include fluorine or trifluoromethylwhich may be used to blue-shift the emission of the complex as disclosedin WO 02/45466, WO 02/44189, US 2002-117662 and US 2002-182441; alkyl oralkoxy groups as disclosed in JP 2002-324679; carbazole which may beused to assist hole transport to the complex when used as an emissivematerial as disclosed in WO 02/81448; bromine, chlorine or iodine whichcan serve to functionalise the ligand for attachment of further groupsas disclosed in WO 02/68435 and EP 1245659; and dendrons which may beused to obtain or enhance solution processability of the metal complexas disclosed in WO 02/66552.

Other ligands suitable for use with d-block elements includediketonates, in particular acetylacetonate (acac); triarylphosphines andpyridine, each of which may be substituted.

The concentration of the phosphorescent light-emitting dopant in thehost material should be such that the film has a high electroluminescentefficiency. If the concentration of the emissive species is too high,quenching of luminescence can occur. A concentration in the range 0.0149wt %, more preferably 0.5-10 wt %, most preferably 1-3 wt % is generallyappropriate.

The host material and the electroluminescent material may be provided asseparate materials as described above. Alternatively, they may becomponents of the same molecule. For example, a phosphorescent metalcomplex may be provided as repeat unit, sidechain substituent orend-group of a host polymer as disclosed in, for example, WO 02/31896,WO 03/001616, WO 03/018653 and EP 1245659. Likewise, a “small molecule”host material may be bound directly to a ligand of a phosphorescentmetal complex.

The phosphorescent material is typically employed as a dopant within thehost material at low (ca. 1-5%) concentration. It will therefore beappreciated that solubility of the phosphorescent material in a givensolvent may be correspondingly low. Alternatively, a phosphorescentmaterial having enhanced solubility may be employed. Such phosphorescentmaterials include complexes bearing alkyl or alkoxy groups as disclosedin JP 2002-324679; dendrimers as disclosed in WO 02/66552: andphosphorescent complexes provided as repeat units, sidechainsubstituents or end-groups of soluble polymers as disclosed in WO02/31896, WO 03/001616, WO 03/018653 and EP 1245659.

Suitable solution processing techniques for deposition of the holetransporting layer and the electroluminescent layer includespin-coating, inkjet printing as disclosed in EP 0880303, laser transferas described in EP 0851714, flexographic printing, screen printing anddoctor blade coating. Provision of such solubilising groups as describedabove may be particularly advantageous for rendering the phosphorescentmaterial more suitable for solution processing techniques such as inkjetprinting.

Following the aforementioned treatment, the hole transporting layer maybe entirely insoluble or partially insoluble. In the case where the holetransporting material is substantially free of cross-linkable groups,the degree of insolubility may vary with the duration and/or temperatureof heat treatment. Any soluble hole transporting material may be removedfrom the layer by rinsing in an appropriate solvent in order to leave anentirely insoluble layer, however the invention also encompassesdeposition of the electroluminescent layer wherein the first layer hasbeen rendered only partially insoluble and the soluble portion has notbeen removed. In this instance there will be some blending between thematerials of the hole transporting layer and the electroluminescentlayer. This may be advantageous in both providing a hole transportinglayer and an electroluminescent layer comprising some hole transportingmaterial, thus creating a gradient of increasing concentration of holetransporting material in moving from the cathode towards the anode.

The hole transporting material, the host material and the phosphorescentmaterial according to the invention must be at least partially solublein a common solvent. Suitable solvents for deposition of theelectroluminescent layer and (where solution deposited) the holetransport layer will be apparent to the skilled person. For example,alkyl- or alkoxy-substituted polyarylenes such as polyfluorenes,polyphenylenes and polyindenofluorenes are typically soluble in aromatichydrocarbons, more preferably mono- or poly-alkylated or alkoxylatedbenzene such as toluene ethylbenzene and cyclohexylbenzene;polyalkylbenzenes, for example xylene, trimethylbenzene andtetramethylbenzene; alkoxybenzenes such as anisole; and blends thereof.On the other hand, PVK is soluble in solvents such as1,2-dichloroethane.

The described treatment has been found to improve efficiency andlifetime of PLEDs. Without wishing to be bound by any theory, thefollowing factors may contribute to these observed increases:

-   -   The first layer may act as a hole transporting, electron        blocking and/or exciton blocking layer when located between the        anode and the second layer.    -   Where a PEDT/PSS layer is used, the insoluble hole transporting        layer may prevent ingress of protons from the acidic PSS        material into the second layer. This may apply in particular        where the polymer comprises basic units such as amines of        formulae 1-6 or Het groups described above, such as pyridyl,        that are capable of accepting protons.    -   The thinness of the insoluble layer, which is achievable in        particular when rinsing of the hole transporting layer is        employed to remove remaining soluble material, may block        electrons from entering the PEDT/PSS hole injection layer and/or        ITO anode layer without any detriment to device performance that        may result from a thicker layer, such as higher drive voltage.

EXAMPLES General Procedure

The invention is exemplified here using the polymer “F8-TFB”,illustrated below and disclosed in WO 99/54385, as the hole transportinglayer.

The general procedure follows the steps outlined below:

-   1) Depositing PEDT/PSS (available from H C Starck of Leverkusen,    Germany as Baytron P®) onto indium tin oxide supported on a glass    substrate (available from Applied Films, Colorado, USA) by spin    coating.-   2) Depositing a hole transporting layer by spin coating F8-TFB from    a xylene solution having a concentration of 2% w/v.-   3) Heating the hole transporting layer in an inert (nitrogen)    environment.-   4) Optionally spin-rinsing the substrate in xylene to remove any    remaining soluble F8-TFB.-   5) Depositing the electroluminescent layer by spin-coating from    xylene solution.-   6) Depositing over the electroluminescent layer a cathode comprising    a first layer of lithium fluoride (4 nm), a second layer calcium (10    nm) and a capping layer of aluminium (>200 nm) as described in WO    00/48258.-   7) Encapsulation of the device using an airtight metal enclosure    available from Saes Getters SpA.

Parameters within this general process may be varied. In particular: theconcentration of the hole transporting material may be up to around 3%w/v. Alternatively, it may be as low as 0.5% w/v to provide aparticularly thin film; the optional heating step may last for anylength of time up to around 2 hours; and the optional heating step maybe at any temperature up to around 2208° C., but preferably above theglass transition temperature of the deposited polymer. As will beapparent to the skilled person, first and/or second polymers, and otherdevice components such as PEDT/PSS, will be subject to thermaldegradation if the heating temperature is excessive, and the heatingtemperature should be selected accordingly.

Example 1

The general procedure above was followed using an electroluminescentlayer comprising:

-   -   a host polymer having the composition 70%        9,9-dioctylfluorene-2,7-diyl, 10% 9,9-diphenylfluorene-2,7-diyl,        10% “TFB” repeat unit and 10% “PFB” repeat unit as disclosed in        WO 02/92723 (TFB and PFB repeat units are illustrated below).        The polymer was prepared by Suzuki polymerisation as disclosed        in, for example, WO 00/53656.

the red phosphorescent dopant platinum octaethylporphine (PtOEP) asdisclosed in Nature (London), 1998, 395, 151.

The host:dopant ratio was 98.8:1.2.

The hole transporting layer was subjected to heat treatment at 130° C.for 10 minutes prior to deposition of the electroluminescent layer.

Comparative Example 1

For the purpose of comparison, a device was made in accordance withExample 1 except that the hole transporting layer (i.e. steps 24 of theabove general method) was omitted.

Example 2

A device was made in accordance with Example 1 except that thehost:dopant ratio was 98.0:2.0.

Comparative Example 2

For the purpose of comparison, a device was made in accordance withExample 2 except that the hole transporting layer (i.e. steps 24 of theabove general method) was omitted.

Example 3

A device was made in accordance with Example 1 except that theelectroluminescent layer consisted of host material CBP and the greenelectroluminescent dendrimer dopant ED1, disclosed in WO 02/066552 andillustrated below, deposited by spin-coating from a chloroform solutionhaving a CBP:ED1 ratio of 8:2.

Comparative Example 3

For the purpose of comparison, a device was made in accordance withExample 3 except that the hole transporting layer (i.e. steps 2-4 of theabove general method) was omitted.

Table 1 below shows a comparison of quantum efficiency and lifetime ofdevices according to the invention and the comparative examples. As canbe seen, there is a very significant improvement in efficiency uponinclusion of the hole transporting layer for Examples 1 and 2.Furthermore, lifetime is improved for both the red PtOEP emitter(Example 1) and the green ED1 emitter (Example 3).

In contrast to Examples 1 and 2, efficiency falls for Example 3 (ED1dopant) relative to its Comparative Example. Without wishing to be boundby any theory, this is believed to be due to quenching of luminance byhole transporting layer 4 in Example 3 as a result of the bandgap ofthis material being insufficiently wide as compared to the bandgap ofthe electroluminescent material. It is believed that the increase inlifetime of Example 3 may be achieved whilst retaining or exceeding theefficiency of the Comparative Example by use of an appropriate widerbandgap material (i.e. an exciton blocking layer as hereinbeforedescribed).

TABLE 1 Quantum Maximum efficiency quantum Device Lifetime* at 100 cd/m²efficiency Comparative Example 1 3852¹ 1.88 2.26 Example 1 10066¹  3.253.52 Comparative Example 2 1.92 2.22 Example 2 2.98 3.11 ComparativeExample 3 670 6.4 6.4 Example 3 1014  3.0 4.2 *“Lifetime” in Table 1 isthe time taken for luminance of the device to halve when driven at aconstant current from a starting luminance of 100 cd/m² in the case ofExample 1 and 400 cd/m² in the case of Example 3. ¹Average of 2 devicesmeasured

Although the present invention has been described in terms of specificexemplary embodiments, it will be appreciated that variousmodifications, alterations and/or combinations of features disclosedherein will be apparent to those skilled in the art without departingfrom the spirit and scope of the invention as set forth in the followingclaims.

1. A method of forming an organic light emitting diode comprising thesteps of: providing a substrate comprising a first electrode forinjection of charge carriers of a first type forming a chargetransporting layer by depositing onto the substrate a chargetransporting material for transporting charge carriers of the firsttype, the charge transporting material being soluble in a solvent;treating the charge transporting layer to render it insoluble in thesolvent; forming an electroluminescent layer by depositing onto thecharge transporting layer a composition comprising the solvent, aphosphorescent material, and a host material; and depositing onto theelectroluminescent layer a second electrode for injection of chargecarriers of a second type.
 2. A method according to claim 1 wherein thefirst electrode is an anode; the second electrode is a cathode; thecharge carriers of the first type are holes; and the charge carriers ofthe second type are electrons.
 3. A method according to claim 1 whereinthe charge transporting material comprises a cross-linkable material andtreating comprises subjecting the charge transporting layer to heat orelectromagnetic radiation in order to cross-link the charge transportingmaterial
 4. A method according to claim 1 wherein the chargetransporting layer is substantially free of cross-linkable groups andtreating comprises subjecting the charge transporting layer to heat. 5.A method according to claim 1 wherein the charge transporting materialis a polymer.
 6. A method according to claim 5 wherein the polymercomprises an optionally substituted triarylamine repeat unit.
 7. Amethod according to claim 6 wherein the triarylamine repeat unitcomprises an optionally substituted repeat unit of formula (I):

wherein each Ar¹, Ar² and Ar³ is the same or different and independentlyrepresents optionally substituted aryl; and n is 0 or
 1. 8. A methodaccording to claim 5 wherein the polymer comprises a repeat unitselected from optionally substituted fluorene, indenofluorene,spirofluorene, and phenylene.
 9. A method according to claim 1 whereinthe phosphorescent material is a metal complex.
 10. A method accordingto claim 1 wherein the host material is a host polymer.
 11. A methodaccording to claim 10 wherein the host polymer comprises a repeat unitas defined in claim
 7. 12. An organic light emitting diode obtainable bythe method according to claim
 1. 13. An organic light emitting diodecomprising, in sequence, an anode; a hole transporting layer; anelectroluminescent layer comprising a phosphorescent material and a hostmaterial; and a cathode, wherein the hole transporting layer is apolymer comprising an optionally substituted repeat unit of formula (I):

wherein each Ar¹, Ar² and Ar³ is the same or different and independentlyrepresents optionally substituted aryl; and n is 0 or
 1. 14. An organiclight emitting diode according to claim 13 wherein the polymer comprisesa repeat unit selected from optionally substituted fluorene,indenofluorene, spirofluorene, and phenylene.
 15. An organic lightemitting diode according to claim 13 wherein a hole injecting layercomprising a conductive organic material is located between the anodeand the hole transporting layer.
 16. An organic light emitting diodeaccording to claim 13 wherein the phosphorescent material is a metalcomplex.
 17. A method according to claim 10 wherein the host polymercomprises a repeat unit as defined in claim
 8. 18. A method according toclaim 1 wherein the charge transporting material is a copolymer.